1. Field of the Invention
The present invention relates to an electrolytic capacitor using a metal having a valvular action, such as aluminum, tantalum, or the like, as the anode, a coating layer formed of an oxide of the valvular metal as the dielectric, and an conducting polymer layer as the cathode. The present invention also relates to a method for manufacturing such an electrolytic capacitor.
2. Prior Art
Customary electrolytic capacitors using a valvular metal, e.g. aluminum, tantalum, are generally fabricated using a valvular metal porous body as an anode element, a coating layer of an oxide of the valvular metal which is formed as the dielectric layer on the porous surfaces and external surface of the porous body, and an liquid electrolyte or inorganic solid electrolyte as the cathode. In addition to the above, a current collector connecting to the anode and the cathode and finally a casing are formed. As the cathode, for example, an organic solvent including an organic acid or the like is used in the aluminum electrolytic capacitor and manganese dioxide or the like for the tantalum electrolytic capacitor.
High responsiveness at high frequencies has been demanded of electronic parts along with the digitization of circuits. It is therefore required for the electrolytic capacitors to be improved in the high responsiveness at high frequencies by a reduction in electric resistance passing through the capacitor. In this situation, the use of a highly conducting polymer as a solid electrolyte for the cathode has been studied and developed.
The electrolytic capacitor has the structural feature in which an oxide layer is formed on the surface of pores of porous valvular metal as the dielectric layer and the valvular metal remaining as the core is used as the anode. When forming the cathode in the capacitor element, it is therefore necessary to coat the surface of very intricate internal pores of the porous element with a conducting polymer in an efficient manner.
In the case of producing a solid electrolyte for the cathode from the conducting polymer by an electrolytic oxidation polymerization method, the following method is used to coat, with the conducting polymer, the surface of the dielectric layer formed on the pore surfaces of the very intricate porous element in an efficient manner. Specifically, a conducting precoat layer is formed in advance on the surface of the insulating dielectric layer. An electrode used for electrolytic oxidation polymerization is allowed to be in contact with the surface of the precoat layer. A solution containing a monomer to be polymerized into an conducting polymer is introduced. An conducting polymer layer is formed by polymerization on the entire surface of the dielectric using the precoat layer as the anode. Then, the electrode for electrolytic oxidation polymerization is removed. On the other hand, as the chemical oxidation polymerization method is utilized, a monomer is allowed to be in contact with an oxidizer capable of oxidizing and polymerizing the monomer on the coating of the dielectric layer to form an conducting polymer layer on the entire surface of the dielectric layer.
In the case where the cathode electrolyte is a solid, a method has been adopted in which an conducting adhesive layers consisting of a carbon paste layer and particulate silver paste layer is interposed between the solid electrolyte layer (manganese dioxide layer or conducting polymer layer) and a current collector for cathode to combine the electrolyte layer with the collector metal.
Such a method is disclosed in Japanese Patent Publication JP-A 6-168855, in which the current collector for cathode is disposed close to the periphery of the valvular metal element. In case of, for example, a aluminum-laminating capacitor, the current collector for cathode is disposed close to any one or all of the side faces and upper and lower faces of the laminated body as shown in FIG. 13.
There have been used a method of disposing the cathode current collectors between the aluminum layers in the aluminum laminated capacitors. Japanese Patent Publication JP-A 4-306427 discloses a method of joining the cathode current collector directly to the conducting polymer layer as a anode.
The aforementioned capacitors having such a structure can only insufficiently reduce in impedance even if it uses a highly conducting polymer as the cathode and hence poses the problem of low responsiveness at high frequencies. This is because various layers are interposed to combine the current collector for cathode with the conducting polymer and also because the surface area of the current collector for cathode is small so that the surface contact resistance cannot be reduced. This is because the natural oxide coating is formed on the surface of the current collector, causing the surface not to reduce in contact resistance.
An object of the present invention is to provide an electrolytic capacitor capable of reducing in resistance of the junction between the current collector for cathode and the conducting polymer, and ensuring high responsiveness of the capacitor at high frequencies.
Another object of the present invention is to miniaturize the capacitor having such characteristics.
The above objects of the present invention can be attained by the provision of an electrolytic capacitor in which the cathode current collector has a roughened surface, a thin carbon layer on the surface, or carbon particles in the surface embedded in the surface, at least on the side facing the conducting polymer layer.
The capacitor of the invention comprises an anode formed of a valvular metal into a porous body, a dielectric oxide layer formed on the entire surfaces of the valvular metal porous body including the pore surfaces, a cathode composed of an conducting polymer layer formed on the dielectric oxide layer, an anode current collector which is electrically connected to an internal metal section of the body, and a cathode current collector which is electrically connected to the cathode, wherein the cathode current collector is a plate or foil of metal provided with the said surface properties and physically joined directly to the conducting polymer layer.
Particularly, in this structure in which such a cathode current collector is directly joined to the dielectric oxide layer using an conducting polymer without interposing various combining layers, such as a carbon paste layer and a silver paste layer, between the cathode current collector and the conducting polymer layer is intended to reduce the contact resistance between them thereby reducing the total impedance and miniaturizing the capacitor.
In the electrolytic capacitor of the present invention, the cathode current collector may be provided with a elastic film of rubber or resin the on the face opposite to the side facing the valvular metal porous body. This structure makes it possible to relieve a stress applied after the formation of the conducting polymer layer whereby a highly reliable capacitor reduced in the occurrence of a short of a circuit and the like.
In the electrolytic capacitor of the present invention, the cathode current collector may be produced by forming a metal thin film on a plastic film. This gives an effective result that the impedance is reduced and that if the circuit would short, a metal thin film of the shorted circuit would vanish whereby the original characteristics would be restored.
In the electrolytic capacitor of the present invention, the cathode current collector may be a part of a casing for packaging a capacitor element. This structure serves to miniaturize the capacitor.
In the electrolytic capacitor of the present invention, the valvular metal porous body of the anode is a porous valvular metal foil and the cathode is disposed so as to oppose one face or both faces of the valvular metal foil and is laminated or wound up. This structure in which the cathode current collector is disposed close to the valvular metal foil for anode ensures that, in addition to the above effect, the collecting area is enlarged, the conducting path of the conducting polymer can be shortened, and the impedance can be reduced.
In the electrolytic capacitor, the porous valvular metal foil as a anode is disposed so as to face both surfaces of the cathode current collector and is laminated or wound. This structure ensures that the impedance can be reduced and the volumetric capacitance can be increased.
In the electrolytic capacitor of the present invention, the cathode current collector may be formed with a number of through-holes penetrating from the surface to backface of a plate or foil, and the valvular metal foil for the anode may be formed with a number of through holes penetrating through the foil. Any one of these structures enables it possible to improve the adhesion of the conducting polymer layer to the cathode current collector or the anode foil and thereby to provide a highly reliable capacitor.
According to a further aspect of the present invention, there is provided a method for manufacturing an electrolytic capacitor comprising of:
a step of forming a dielectric oxide layer on the surface of a porous body formed of a valvular metal and the entire surface of pores;
a step of mounting the porous body on a metal sheet of the cathode current collector having a roughened surface, a thin carbon layer on the surface, or carbon particles in the surface embedded in the surface, at least on the side facing the conducting polymer layer; and
a step of forming an conducting polymer layer on the dielectric layer. In the invention, the valvular metal porous body may have any one of a foil form and a block form (including a laminated and wound types).
The space between the cathode current collector and the dielectric oxide layer can be filled only with the conducting polymer layer by forming the conducting polymer layer after the valvular metal porous body mounted on the cathode current collector.
In the invention, the valvular metal porous body on which the conducting polymer layer is formed in advance may be mounted on the cathode current collector.
In the method for manufacturing of the present invention, an conducting polymer layer may be further interposed between the porous body and the cathode current collector after the step of mounting, on the cathode current collector, the valvular metal porous body on which the conducting polymer layer is formed. The adhesion of the cathode current collector to the conducting polymer formed on the valvular metal porous body is thereby improved contributing to a reduction in the impedance.
The method for manufacturing the electrolytic capacitor of the present invention may include the following method: Specifically, the cathode current collector is a valvular metal foil provided with an oxide layer. The valvular metal anode foil and the cathode foil are integrated into a structure by laminating or winding up the both foils via a separator. An electrode for electrolytic oxidation polymerization is disposed on the entire end perpendicular to the both foils and the separator. An conducting polymer is allowed to grow inside the structure from the electrode by electrolytic oxidation polymerization to coat the inside of pores of the porous structure. A uniform conducting polymer layer can be simply formed by electrolytic oxidation polymerization.